1. Field of the Invention
The invention relates to cabinets for outdoor use for enclosing racks of electronics equipment, and to methods of manufacturing such cabinets.
2. Background Art
It is known to provide outdoor cabinets for communications equipment such as racks of cards holding electronic or optical components. Such cabinets are provided at outdoor locations where there is no suitable existing building to hold the equipment and protect it from adverse environmental conditions, and where the size of the equipment is too small to justify a dedicated building. Examples of such cabinets include roadside cabinets for housing electronics for controlling traffic signals, or cabinets for cellular radio network base station electronics.
Some of the principal considerations in the design of such cabinets include: level of environmental protection, corrosion resistance, strength to cost ratio, thermal management, public safety, aesthetic considerations, vulnerability to vandalism, ease of installation, ease of access to equipment, and level of electromagnetic shielding. The issue of thermal management will be discussed in more detail.
The main thermal effects are as follows. The equipment inside the enclosure generates heat which may need to be extracted. The enclosure may be subject to external sources of heat, such as solar radiation.
A common constraint in the design is that the equipment should not have ambient air passing over electronic components, to avoid deterioration from contaminants and corrosives in the ambient air. Thus the equipment may be sealed within the enclosure, and heat from components within the sealed part may be conducted to the outside of the sealed part, and the sealed part cooled by fans circulating ambient air. A heat exchanger may be used if further cooling is required. In extreme cases, air conditioning may be appropriate.
It is known to provide a cabinet with a single metal skin for the sides. This can provide sufficient strength and weatherproofing, to resist wind and rain. It offers little resistance to solar radiation, even if painted a light colour or silvered to reduce heat absorption.
It is also known from U.S. Pat. No. 4,535,386 to provide a sealed enclosure for electronic components such as silicon controlled rectifiers, using a single metal skin, but enhancing the natural convection past heat sinks mounted to protrude through the skin, by providing an outer chimney surrounding the heat sinks to duct cool air past the sinks and improve dissipation. However this disclosure is not concerned with a cabinet for outdoor use.
The provision of a double skin for the sides of the enclosure is also known, which assists in cooling the equipment by the "cold wall cooling" principle. A chimney effect is produced in the cavity between the skins. An inlet at the bottom of the cavity allows air in, to flow up the cavity and out of an outlet at the top of the cavity. Heat from the internal skin is thus extracted from the cabinet by convection, and thus the equipment inside the cabinet can be kept cool. Furthermore, heat from solar radiation on the external skin can also be extracted, and prevented from reaching the equipment inside.
Where the internal equipment is producing more heat than can be handled by natural convection in the double skin, it is known to provide fans at the top of the cavity to draw air through the cavity and thus increase the rate of heat extraction. If this is still insufficient, an air to air heat exchanger could be provided, or an air conditioning unit might be considered to take the air within the enclosure and return it to the enclosure at a lower temperature.
It is appreciated that such active cooling measures may reduce the reliability of the cabinet, or increase it's maintenance costs, and will certainly increase it's production cost. Accordingly thermal and fan management systems are used to minimise the amount of time the active cooling measures are used, to maximise the life expectancy of fans and associated filters.